A cathode ray tube (CRT) and any other device requiring an electron beam normally contains a hot filament to cause thermionic emission from a cathode. There has long been an interest in developing cold cathodes, depending on field emission of electrons, to replace the hot cathodes. For low current devices, such as scanning electron microscopes, there are a large number of patents describing field emission electron guns. For higher current applications, such as TV displays, prior art field emission cathodes, generally based on molybdenum and silicon, have not proven sufficiently robust for commercial applications. Tip damage occurs from ion back scattering caused by the presence of background gases and the tips fail when driven at high current densities.
It has been demonstrated that carbon-based microtip cathodes can be fabricated and used as a replacement for the molybdenum- or silicon-based microtip field emission cathodes. It has also been demonstrated that the diamond can be monolithically integrated with gated electrodes in a self-aligned structure, using integrated circuit fabrication techniques ("Advanced CVD Diamond Microtip Devices for Extreme Applications," Mat. Res. Soc. Symp. Proc., Vol. 509 (1998).
Extraction of electrons from cold electron-emissive material by a gate electrode has been widely studied in recent years. Much of the work in cathode development was directed to electron sources for use in flat panel displays. U.S. Pat. No. 3,753,022 discloses a miniature directed electron beam source with several deposited layers of insulator and conductor for focusing and deflecting the electron beam. The deposited layers have a column etched through them to the point field emission source. The device is fabricated by material deposition techniques. U.S. Pat. No. 4,178,531 discloses a cathode ray tube having a field emission cathode. The cathode comprises a plurality of spaced, pointed protuberances, each protuberance having its own field emission producing electrode. Focusing electrodes are used to produce a beam. The structure produces a plurality of modulated beams that are projected as a bundle in substantially parallel paths to be focused on and scanned over the screen of a CRT. Manufacture using a photo resist or thermal resist layer is disclosed. U.S. Pat. No. 5,430,347 discloses a cold cathode field emission device having an electrostatic lens as an integral part of the device. The electrostatic lens has an aperture differing in size from the first size of the aperture of the gate electrode. The electrostatic lens system is said to provide an electron beam cross-section such that a pixel size of from approximately 2 to 25 microns may be employed. Computer model representations of the side elevation view of prior art electron emitters are shown.
Among relatively recent patents, U.S. Pat. No. 5,719,477 discloses conically shaped electron emitters wherein a control voltage can be applied independently to each group of the plurality of groups of cathodes and also to the gate electrodes. U.S. Pat. No. 5,723,867 discloses a gate electrode with the emissive surface in a cone recess with focusing electrodes on the surface above the recess. In one embodiment there is a "shield electrode." U.S. Pat. No. 5,814,931 likewise has the emitter in a "hollow" and focusing electrodes in four parts around the plurality of emitters. The emitter is a refractory metal such as tungsten. The focusing voltage varies during the scan angle when the electron emitter is used in a CRT. The focusing is designed to be more intense when the electron beam is in the peripheral part of a screen. Dividing the emitter electrode is also disclosed. U.S. Pat. No. 5,850,120 discloses a method of obtaining linearity in brightness while using an emitter following the Fowler-Nordheim type emission current. A secondary gate electrode has lower potential than a first gated electrode and the voltage between the cathode and the secondary gate electrode is proportional to the voltage between the cathode and the primary gate electrode. A ternary gate electrode is also disclosed, which is at a higher voltage to increase current and prevent secondary gate current.
Publication No. 09306376 from the Japanese Patent Office discloses electron beams emitted from conical electron sources and focused by a first focus electrode and accelerated by a second focusing electrode. Independent electric potentials of the focusing electrode and an anode are used to form a focus on a screen with a main lens, which is a conventional bipotential lens.
The book Basics of Electron Optics describes the principles of electron lenses, the factors limiting the quality of electron optics and, in Ch. 11, electron guns based on conventional hot cathodes that are used in television and other CRTs. In addition to the electron gun, which forms and focusses a beam, there is a drift region that brings the beam to a spot on the screen and a deflector or yoke that deflects the beam. The yoke of a CRT is not a part of this disclosure and will not be discussed further. The referenced book discusses the three regions in a CRT: (1) the beamforming region, which includes the cathode and electron optics lenses, which supplies a divergent beam of electrons; (2) the main lens region, which uses cylinder lenses, usually co-linear, to focus the divergent beam toward the display screen, and (3) the drift region, which is past the neck of the CRT and in which the redirected electrons move, without further forces, toward the screen. In such CRTs, there is a crossover region in the electron beam near the cathode and the beam is smeared by the combined effects of lens aberrations, space charge and thermal distribution of emitted electrons. The result of this smearing is less resolution in the image formed on the screen.
U.S. Pat. No. 5,343,113 discusses the introduction of the laminar flow electron gun, which produces a clearer, brighter display than the crossover guns. In a laminar flow gun, the electrons emitted from the cathode tend to flow in streamline paths until they are converged to a focus at the viewing screen. This patent, typical of field emission electron guns, discloses use of several lenses along the electron beam. The lenses significantly extend the required length of the gun.
What is needed is an electron gun having a cold cathode that has a long lifetime without requiring an ultra high-vacuum operating environment and having a lens arrangement that allows for a compact configuration and sufficiently high current in a small spot for many CRT applications, including TV.